1. Field of the Invention
The present invention relates to an oscillation frequency control circuit which constantly and precisely controls the oscillation frequency of a horizontal oscillation circuit of a display unit in accordance with the frequency of an input horizontal synchronizing signal.
2. Description of Related Art
FIG. 1 schematically designates a block diagram of a conventional oscillation frequency control circuit in conjunction with a peripheral circuit available for a display unit. The reference numeral 21 shown in FIG. 1 designates a one-chip microcomputer incorporating a counter, a timer, a ROM, and a RAM. The microcomputer 21 further incorporates a pair of input terminals H and V and three output terminals DA1 through DA3. These input terminals H and V respectively receive horizontal synchronizing signal HS and vertical synchronizing signal VS. The horizontal synchronization signal HS is also routed to a horizontal synchronizing circuit 31 via a switch 30.
Those output terminals DA1 through DA3 of the microcomputer 21 are respectively connected to input terminals CLK, DATA and LOAD of a digital-to-analog (D/A) converter IC 22. An inverted input terminal of a first operation amplifier 23 is connected to an output terminal OUT of the D/A converter IC 22 via a resistor 24. A resistor 25 is connected between the inverted input terminal and an output terminal of the first operation amplifier 23. The inverted input terminal of the first operation amplifier 23 is connected to a power-supply source Vcc via a variable resistor 26, whereas a non-inverted input terminal of the first operation amplifier 23 is connected to a reference power-supply source Vref.
The output terminal of the first operation amplifier 23 is connected to an inverted input terminal of a second operation amplifier 27 via a resistor 28. A variable resistor 29 is connected between the inverted input terminal and an output terminal of the second operation amplifier 27, whereas a non-inverted input terminal of the second operation amplifier 27 is connected to the reference power-supply source Vref. The output terminal of the second operation amplifier 27 is connected to a horizontal oscillation circuit 32.
Next, referring to the flowchart shown in FIG. 2, operation programmed by the microcomputer 21 is described below.
When the power is ON, program of the microcomputer 21 is activated and the initial step S21 is entered, in which horizontal synchronizing signal HS received via the input terminal H is counted for a predetermined period of time from the rear edge of vertical synchronizing signal VS received by the other input terminal V. FIG. 3 presents a table designating the corresponding relationship between the counted number Fh and a frequency control data DA. The data shown in this table is previously stored in the ROM of the microcomputer 21.
Next, step S22 is entered, in which the counted number is converted into a control data as per the table shown in FIG. 3, and then, step S23 is entered, in which the digital controlled data is supplied from those output terminals DA1 through DA3 to the D/A converter IC 22, and then, the analog-converted signal is supplied to the horizontal oscillation circuit 32 via the first and second operation amplifiers 23 and 27, and then the oscillation frequency of the horizontal oscillation circuit 32 is properly controlled in accordance with the frequency of the horizontal sychronizing signal HS.
It is ideal to arrange that frequency Fi of the input horizontal synchronizing signal HS and the output oscillation frequency Fo can enter into a relationship shown in FIG. 4 with a solid line. Those terms "Fimax" and "Fimin" respectively designate the maximum and minimum values of frequency range of the horizontal synchronizing signal HS. The oscillation frequency is properly controlled in order that the relationship Fomax=Fimax and Fomin=Fimin can be held constant.
Nevertheless, when individually operating respective products, due to unevenness of the constant in the oscillation circuits, deviation is unavoidably generated as shown in FIG. 4 with a broken line. To compensate for this, independent of the normal operation described above, an extra corrective operation for correcting the deviation must be executed. Those processes needed for correcting this deviation are described below.
Normally, the operation for correcting the above deviation is executed by applying a pair of variable resistors 26 and 29. FIG. 5 designates the input and output characteristics of the first and second operation amplifiers 23 and 27, where Vi designates input voltage and Vo output voltage. To correct the above deviation, initially, the switch 30 is released to prevent the horizontal synchronizing signal HS from being supplied to the horizontal synchronization circuit 31. Next, the horizontal synchronizing signal HS of the minimum frequency Fimin is supplied to the microcomputer 21, and then operates the variable resistor 26 so that the oscillation frequency can be equal to the Fomin, thus correcting the characteristic shown in FIG. 5. And, the horizontal synchronizing signal HS of the maximum frequency Fimax is supplied to the microcomputer 21, and then operates the variable resistor 2a so that the oscillation frequency can be equal to Fomax, thus correcting the characteristic shown in FIG. 5. After repeatedly executing these processes for several rounds, a desired characteristic of the oscillation frequency can be achieved.
Nevertheless, since any those conventional circuits for controlling the oscillation frequency is structured as described above, in order to fully eliminate unevenness of the circuit constant, any conventional device needs to install a circuit incorporating a plurality of operation amplifiers, and yet, the corrective operation must be executed via an external source. In consequence, circuit structure involves complexity, and yet, execution of the corrective process is annoyingly intricate and troublesome for the concerned.